Polymer electrolyte fuel cells that use proton-conductive solid polymer membranes operate at low temperatures compared to other types of fuel cells such as solid oxide fuel cells and molten carbonate fuel cells and are considered as a prospective candidate for stationary power sources and mobile power sources for automobiles and the like. Practical application thereof has already started.
Expensive metal catalysts such as platinum (Pt) and platinum alloys are used in polymer electrolyte fuel cells and this has raised the price of these fuel cells. Accordingly, development of a technology that decreases the amounts of these noble metal catalysts used and reduces the cost of these fuel cells has been desired.
For example, Japanese Unexamined Patent Application Publication No. 2007-250274 discloses an electrocatalyst for the above-described fuel cells, in which the average particle size of a catalytic metal is increased to be larger than the average pore size of fine pores of a conductive carrier so that the catalytic particles do not penetrate the fine pores. As a result, the amount of unused catalytic metal that does not contact the electrolyte polymer is decreased and the efficiency of utilizing expensive catalytic metals is enhanced.
However, according to the approach of enhancing the efficiency by increasing the size of the catalytic metal particles, the specific surface area will be decreased as the percentage of the catalytic metal particles contacting the electrolyte polymers is increased. In other words, the power generating performance per catalytic metal weight is degraded and the amount of expensive metal catalysts needs to be increased in order to maintain the same power generating performance as before this approach is taken.